Coupling arrangement for concentric transmission line



C. R. ELLIS March 15, 1960 COUPLING ARRANGEMENT FOR CONCENTRIC TRANSMISSION LINE Filed April 15, 1955 FlG.2b.

y a 5 20 22 a H m w G b H llm-| 1| 1 W- 3 7 Z l I 2, DH T' 'X Y m. m. TL "w 9 p uunullHhuh ii 1.1[ .1; F a 1 |1|l m 8 m I: I. 1| 2 Q G F ililllillllll' 7 h AAAAAAAAAAAAAAAAA 1 INVENTOR CHARLES R.ELL|S BY 2 HIS AGENT.

, aerate? Patented Mar. 15, 1960 COUPLING ARRANGEIHENT FOR CONCENTRIC TRANSMISSION LINE Charles Richard Ellis, Syracuse, N.Y., assignor to General Electric Company, a corporation of New York Application April 15, 1955, Serial No. 501,695

3 Claims. (Cl. 333-82) This invention relates to space resonant cavities of the concentric transmission line type and in particular to arrangements for coupling energy in and out of space resonant cavities.

It is known that a space resonant cavity of the type comprising a tubular outer conductor and a centrally disposed inner conductor may be excited by suitable exciting means such as an electron discharge device, to establish electro-magnetic waves between the inner and outer conductors. The frequency of this space resonant system just defined may be adjusted by adjustment of the electrical length of the concentric transmission line. In the past, coupling arrangements for extracting energy from the electromagnetic field established within the resonator have exhibited certain undesirable features. For example, when a probe, such as of the capacitor type, is introduced into the cavity for the purposes of extracting energy, a symmetrical discontinuity is introduced in the electromagnetic field causing undesirable standing waves to appear. If the cavity is operating as part of an oscillator, the unsymmetrical coupling results in undesirable modes of oscillation. If the cavity is operating as part of an amplifier circuit, the result is inefficient coupling of energy.

It is therefore an object of my invention to provide a new and improved coupling arrangement for a concentric transmission line resonator in which optimum operation of the resonator is obtained over a wide band of frequencies.

It is another object of this invention to provide an improved coupling arrangement for resonant cavities of the concentric transmission line type.

It is another object of this invention to provide an improved concentric transmission line type cavity resonator arrangement.

It is a further object of this invention to provide an improved electro-magnetic wave coupling arrangement.

It is a further object of this invention to provide a combined wave coupling, resonator tuning and load matching arrangement for use with coaxial cavity resonators.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings, in which:

Fig. l is an illustration in transverse cross sectional form, of a prior art configuration for coupling electromagnetic waves from a coaxial transmission line type cavity resonator;

Fig. 2a is a longitudinal view of a concentric transmission line cavity resonator embodying one of my improved coupling arrangements;

Fig. 2b is an illustration in transverse cross sectional form of the present invention.

Fig. 3a is a longitudinal sectional view of a concentric transmission line cavity resonator embodying a further embodiment of my invention providing symmetrical coupling and improved load matching.

Fig. 3b is a lumped circuit representation of the arrangement of Fig. 3a.

Referring to Fig. 1 there is shown a concentric transmission line cavity resonator comprising an inner conductor 1 and an outer conductor 2. The coaxial transmission line cavity resonator generally comprises a quarter wave length section closed at one end, and coupled at the other end to a suitable generator of radio frequency waves such as an electron discharge device. The quarter wave section is dimensioned to be resonant at the desired operating frequency. In order to couple a desired amountof the electro-magnetic waves developed within the resonator, a suitable coupling arrangement is provided in the form of a loading probe 3, which is spaced from the inner conductor 1 and coupled by meansv of a coaxial cable comprising the inner conductor 4 and the hollow outer conductor 5 to a suitable external load, not shown. Referring to Fig. 1 it should be noted that the effect of the loading probe is to provide an u'nsym' metrically distributed capacity as shown in dotted form at 6. This unsymmetrical distributed capacity results in distorting the electro-magnetic field pattern sufficiently to cause undesirable loading of resonator. If the coaxial resonator is operating as an oscillator, then waves of undesirable frequency may be generated. If the coaxial. cavity resonator is operating as an amplifier, then inefiicient coupling of waves may result.

To avoid the difliculties indicated, applicant has invented an improved coupling arrangement for use with coaxial cavity resonators wherein the improved coupling arrangement results in a more symmetrical distribution of the coupling capacity. Referring to Fig. 2a there is shown a coaxial cavity resonator 7 comprising an inner conductor 8 and an outer conductor 9. The resonator is sealed at One end by the tuning disk 10 and coupled to a source of radio frequency oscillations 11, at the other end. The tuning disk 10 is driven axially to vary the dimension of the coaxial line resonator by means not shown, such that a desired axial resonant mode is established within the resonator. The source 11 in cooperation with the coaxial cavity resonator 7 establishes radio frequency waves within the cavity resonator portion. In order to extract a desired portion of the energy developed in the cavity resonator for coupling to the load circuit 12, a loading ring 13 is provided. This ring cornprises a disk of conductive material such as aluminum or brass which is spaced from the inner conductor 8 and from the outer conductor 9. The loading ring 13 is connected to the load circuit by means of the coaxial cable 14 which comprises an inner conductor 15 and an outer conductor 16. To accommodate various axial positionsof the loading ring, the inner conductor 15 is made telescopic at the one end 17. The disk is moved axially within the cavity resonator by means not shown, in order to couple the desired portion of the energy developed within resonator 7 to the load circuit 12. i

Referring to Fig. 2b it should be noted that by utilizing a loading ring, a symmetrical distribution of coupling capacity results. This eliminates the undesirable effects previously described with respect to the probe type of coupling arrangement. While in Fig. 2a the coaxial transmission line coupling to the loading ring is shown as connected to the bottom of the cavity resonator, it is obvious that under suitable conditions a similar connection can be provided to the upper portion of the ring as shown in dotted form at 18, or through the side by a suitable slot, or through the inner conductor as will be shown in Fig. 3..

For purposes of simplicity identical numerals are rea tained whenever possible in Fig. 3. In this figure, the conductive loading ring 13 is coupled by means of a coaxial transmission line 21 to the load circuit; The coaxial transmission line comprises a shorting disk 19 connected to the outer conductor 29 of the coaxial. line 21 and electrically contacting the hollow conductor 22 by rneans of peripherally mounted spring fingers. The inner conductor 24 of the coaxial transmission line 21 extends through the hollow conductor 22 and electrically contacts the loading ring 13 through a slot provided in the hollow conductor 22. The conductors 22 and 25 form therebetween the coaxial cavity resonator 2d corresponding to 7 of Fig. 2a. In order to achieve the desired amount of capacitive coupling in the cavity resonator 26, the conductive ring 13 is permitted to be moved axially by arm 23 along the inner conductor 24 which is rendered conductively telescopic at one end as shown at 27. As the telescopic inner conductor 24 is adjusted to vary. the distance X, the amount of capacitive coupling is similarly varied. By a separate distinct adjustment, it is possible with the arrangement of Fig. 3 to achieve proper impedance match between the source 11 and the load circuit i2. This is accomplished .by varying the length of the uncovered portion 28 of the coaxial line 21. The distance Y is varied by axially moving the shorting disk 19 Within the conductor 22. To accommodate the axial movement of the shorting disk 19, the portion of the inner conductor 24 is rendered telescop ic as shown at 27.

Referring to Fig. 3b, there is shown a lumped circuit equivalent for the arrangement of Fig. 3a. The load circuit is shown asa resistance element 29. The cavity is shown as the parallel tuned resonant circuit 36 comprising an inductance 31 and a capacitance 32. The amount of coupling varying the amount of radio frequency energy to be coupled from the tuned circuit 36 to the load circuit 29, is controlled by the capacitor 33 which corresponds to the axial position of the loading ring 13. To provide a desirable impedance match between the tuned circuit 30 and load circuit 29, there is provided a variable inductance 34 which corresponds to the portion of the uncovered coaxial transmission line shown at 28 in Fig. 3.

While a specific embodiment has been shown and described it will of course be understood that various modifications may yet be devised by those skilled in the art which will embody the principles of the invention and found in the true spirit and scope thereof.

I claim:

1. An arrangement comprising a coaxial cavity resonator, said resonator comprising an outer conductor and associated inner conductor, an electrical circuit external of said resonator, means for coupling electrical energy between said circuit and said resonator comprising an annular dish centered on said inner conductor and positionable along the length of said inner conductor between said inner and outer conductors, means for positioning said annular disk, exciting means for establishing electromagnetic waves within said resonator, a metallic plunger located within said inner conductor, a coaxial transmission line mounted within said inner conductor, said line comprising an inner and outer conductor, an we tension of said inner conductor of said coaxial line connected through an opening in said plunger and said cavity resonator inner conductor to said disk, and means for coupling said line to said electric circuit.

2. An arrangement comprising a coaxial cavity resonator, said resonator comprising an outer conductor and an associated inner conductor, an electrical circuit external of said resonator, means for coupling electrical energy between said circuit and said resonator comprising an annular probe centered on said inner conductor and positionable along the length of said inner conductor, between said inner and outer conductors, a coaxial transmission line mounted within said inner conductor, means for coupling said line to said electrical circuit, said line comprising an inner and outer conductor, a shorting disc mounted within said coaxial line inner conductor, an extension of said inner conductor of said line connected through an opening in said shorting disc and said cavity resonator inner conductor with said probe, and means to achieve proper impedance match between said coaxial cavity resonator and said electrical circuit comprising means for adjusting the longitudinal position of said shorting disc within said coaxial line inner conductor to vary the length of the extension of said inner conductor of said line.

3. An arrangement comprising a space resonant system of the concentric transmissional line type, said system comprising a tubular outer conductor and a centrally disposed inner conductor, exciting means for establishing electromagnetic waves within said system, means for adjusting the resonance frequency of said system comprising a metallic plunger located Within said space resonant system and positionable along the length of said line, an electrical circuit external of said line, means for coupling said waves to said circuit comp-rising an annular probe, capacitively coupled symmetrically about the longitudinal axis of said line between said inner and outor conductors, a coaxial transmission line comprising an inner conductor and an outer conductor located externally of said system, a longitudinal extension of said coaxial line inner conductor at one end of said coaxial line connected through a slot in said plunger to said probe, means for achieving a proper impedance match between said exciting means and said electrical circuit comprising means for varying the length of said longitudinal extension of said coaxial line inner conductor,

and said coaxial line outer conductor connected to said plunger, and means for connecting said coaxial line to said circuit.

References Cited in the file of this patent UNITED STATES PATENTS 2,227,664 Smith Jan. 7, 1941 2,245,597 Lindenblad June 17, 1941 2,421,591 Bailey June 3, 1947 2,544,502 Preist May 29, 1951 2,771,516 Backsbaum Nov. 20, 1956 2,799,797 Peter July 16, 1957 FOREIGN PATENTS 66,246 Denmark Jan. 12, 1948 

